A student strokes an unmagnetised cobalt rod several times in the same direction with one pole of a permanent magnet. After stroking, the cobalt rod is found to retain its magnetism for a long time. Explain why cobalt can be permanently magnetised and describe what would happen if the north pole of a bar magnet were brought close to one end of the magnetised cobalt rod.

Cobalt is a magnetically hard material, meaning its magnetic domains are strongly aligned and resist demagnetisation. When a magnetic field is applied, the domains align and the material becomes magnetised. Once the external field is removed, the domains remain largely in the same orientation, so the cobalt rod keeps its magnetism for a long time.

If the north pole of a bar magnet is brought close to one end of the magnetised cobalt rod, that end will act as a magnetic pole. If the end of the cobalt rod is a north pole, it will repel the north pole of the bar magnet; if it is a south pole, it will attract the north pole of the bar magnet.

Thus the interaction depends on the relative pole orientations: like poles repel, unlike poles attract.